Apparatus for measuring the phase shift between the input and output voltages of a circuit under test



May 6, 1952 Filed April 12,

c. E. INGALLS 2,595,263 APPARATUS FOR MEASURING THE PHASE SHIFT BETWEEN THE INPUT AND OUTPUT VOLTAGES OF A CIRCUIT UNDER TEST 1946 2 Sl-lEETS-SI-IEET 1 g m L 'Z LE 2 O: E .m E E J Q h. I n (L E 5 5 a '3 LL E II E i E 5,

DC 5 5 B 8 5 g m 5 5 g U U s g 8 INVENTOR CLYDE E. I NGALLS ATTORNEY Filed April 12, 1946 C. APPARATUS FOR MEA E INGALLS SURING THE PHASE SHIFT BETWEEN THE INPUT AND OUTPUT VOLTAGES OF A CIRCUIT UNDER TEST 2 SHEETS-SHEET 2 f 7 LOW PASS A P I V MIXER FILTER M UF ER MIXER AMPLIFIER VARIABLE OSCILLATOR Il FIXED BAND PASS OSCILLATOR FILTER PHASE PHASE SH IF TER DETECTOR INVENTOR CLYDE E. INGALLS AT TOR NEY Patented May 6, 1952 APPARATUS FOR MEASURING THE PHASE SHIFT BETWEEN THE INPUT AND OUTPUT VOLTAGES OF A CIRCUIT UNDER TEST Clyde E. Ingalls, Canisteo, N. Y., assignor, by mesne assignments, to the United States of America as represented by the Secretary of "the Navy Application April 12, 1946, Serial No. 661,600

2 Claims.

This invention relates to electrical indicating and measuring devices and .particularly to apparatus for electrically indicating and measuring the phase difference of two voltage waves or currents having the same frequency.

The knowledge of the phase difference between signals is required in practice for many purposes; for instance, measurement of distances with the aid of reflected waves as in radars, altimeters, and direction finders. In vacuum tube amplifiers the phase difference between the input .and output signals becomes increasingly important as the band pass of the amplifier is increased.

It is an object of this invention to provide novel apparatus with which phase shift of a voltage or current wave of fixed or variable frequency may be rapidly measured.

It is another object of this invention to provide a method and apparatus for simply and accurately measuring and indicating the phase difference of two voltages or current waves of the same frequency, one being the input and the other the output, of a circuit under test.

These and other objects and advantages will suggest themselves to one skilled in the art and will become apparent from the following description of the invention taken in connection with the accompanying drawing in which:

Fig. 1 is a block diagram of an embodiment of the invention; and

Fig. 2 is a block diagram of another embodiment of the invention.

Referring to the drawings, and particularly to Fig. 1, output signals of variable frequency oscillator I are connected to the substantially identical mixers l3 and I3. Fixed frequency oscillator H is connected to mixer l3 and to calibrated phase shifting device l2, preferably a poly-phase condenser and phase-splitting network, and thence to mixer 13'. Substantially identical low pass filters M and [4 are fed by the outputs of mixers l3 and I3 respectively, which outputs are of equal frequency and equal to the difference (or sum) between the frequencies of oscillators I9 and II. The outputs of filters l4 and [4' are amplified by amplifiers l5 and I5 respectively and fed to a conventional type phase detector [8, preferably a cathode ray oscilloscope as diagrammatically illustrated by the deflection plates thereof, through terminal l6 and switch [1.

In operation, consider oscillator Ill adjusted so that its voltage output is E1 cos (wlt-ln) where E1, col, and 51 are respectively the amplitude, angular velocity, and relative phase of the voltage, and the voltage output of oscillator Jl is' E2 =2 cos (w2t+2) when E2, wz and 2 are respectively the amplitude, angular velocity, and relative phase of the voltage. Phase shifter I2, if adjusted to introduce a phase shift, applies the voltage E2 cos (anti-w) to mixer I 3. The voltage, e applied to the grid of mixer I3 is therefore:

and the voltage, e applied to the grid of mixer I3 is:

The plate currents of mixers l3 and I3 are found by substituting the above equation in the following equations:

where ip is the plate current, 2'0 is the'direct current component of the plate current, e is the grid voltage, and e is the grid voltage squared, etc.

Low pass filters l4 and I4 are designed so that they reject direct current, 2'0, in the above equation, and so that the highest frequency passed is lower than the frequency corresponding to the angular velocities in and (v2. The output of filter I4 is then proportional to which is amplified without further change in amplifier l5 and connected to the vertical deflection plates of phase detector [8. Likewise the output of filter I4 is proportional to which isamplified without further change in amplifier i 5 andconnected to the horizontal deflection plates of phase detect-or I8.

The two voltages applied to the phase detector then have the same frequency but diifer in phase by the angle (2'z) which is the phase introduced by phase shifter I2 in the voltage from oscillator II to mixer [3.

As a preliminary adjustment, phase shifter i2 is adjusted until phase detector I8 indicates no difference in phase between the twovoltages fed to it and the calibration 'dial (not shown) on phase shifter I 2 is then set to read zero. The circuit [9 to be tested for amount of phase shift (in this instance an amplifier) is then inserted by changing the position of switch I! so that the signal output of amplifier l5 passes through amplifier l9, and again adjusting phase shifter l2 until phase detector l8 shows no difierence in phase in the two voltages applied to it. The amount of phase shift introduced by inserted circuit I9 is then read directly from the aforementioned calibrated dial on phase shifter l2.

Phase detector 18 is independent of frequency, and thus a circuit may be checked for phase shift over a wide range of frequenciessi'mply by turning variable oscillator It to changethe frequency of the signal fed to the circuits illustrated.

Reference is made to Fig. 2 which is another embodiment of the invention wherein phase detector I8 need not be independent of -frequency. Elements in Fig. 2, which ftmctionas inFig. 1 have been similarly numbered. I

Variable oscillator l and fixe'd'bs'cillatorii are connected to mixer l3, whoseoutput-'-f eeds low pass filter l4 and amplifier i5 as described in Fig. 1. The output of amplifier l5 and variable oscillator iii are connected to mixer 13' *whose output feeds bandpass filter 20. Filter 20 is chosen so that its band pass frequency is the same as the frequency of the oscillations of fixed oscillator H. Calibrated phase shifting device'l 2, preferably a poly-phase 'condenserand phasesplitting network, is connected to'fixed oscillator Phase detector 18 is fed by'phaseshiftern and by mixer [3' through filter 20.

In operation, a signal Whose frequency is equal to the difference of the frequencies of variable oscillator l6 and fixed oscillator II is applied to the circuit under test, amplifier 19, through switch H. The output of amplifier I9 is mixed with the same variable oscillator signal in mixer I3 to produce a voltage whose frequency is the same as fixed oscillator ll. Filter 20 excludes all of the other heterodyned signals from phase "detector 18. Calibrated phase shifter 12 is -thenadjusted so that phase detector It indicates no'phase difference between the two signals connected toit. The amount of phase shift introduced by amplifier I9 is then read from the calibrated dial," not shown, of phase shifter 12.

Phase shift for comparison purposes and ph'ase detection for indicating purposes is'thus accomplished at a constant frequency irrespective of the frequency of the test signal applied tothecircuit under test, amplifier'lfi in this case. A

The circuits of this invention may be --used as an automatic range computer of the phase shift type in radar systems bysubstituting a rectifier type of phase detector for detector l8. whose output feeds a conventional servo mechanism for continuously positioning phase shifter l2 and the range dials of the radar.

Although the drawing and specification show what is now considered to bethe most desirable arrangement, it will be apparent to one-skilled-in the art that various applications, changes, and modifications may be made therein' without'departing from the scope of 'the'mvennon as -'set forth in the appended claims.

What is claimed is:

1. Apparatus for measuring the phase shift between the input and output voltages-of a circuit comprising, a fixed frequency oscillator, a variable frequency oscillator, first and second mixer circuits, means for applying; the'output of said variable frequency oscillator directly to said first and second mixers, an "adjustable phase shifter, means for applying a portion-of the output of said fixed frequency oscillator through said phase shifter to said second mixer, means for applying another portion of the output of said fixed frequency oscillator directly to said first mixer, first and second filters respectively coupled to said first and second mixers for selectively passing signals of the same frequency, a phase detector, means for applying the signals from 'said first and second filters to said phase detector whereby an initial indication of the difference in phase of'said signals may be obtained, and

means 'for disconnecting said second filter from said phase detector and connecting said circuit under test between said second filter and said phase detector, whereby the adjustment of said phas'eshifter to obtain an indication corresponding to said initial indication is a measure of the phase shift introduced by said circuit.

2. Apparatus for measuring the phase shift between the input and output voltages of a circuit under test comprising, a fixed frequency oscillator, a Variable frequency oscillaton an adjustable phase shifter, first means for combining a portion of the output from each of 'said fixed and variable frequency oscillators to obtain signals having frequencies equal to -the-sumand difference of the frequencies of said 'variable'frequency oscillator and said fixed frequency oscillator, second'means for combining a portion ofthe output of'sa'id variable frequency oscillator anda portion of the output of said fixed frequency oscillator shifted in phase by said adjustable phase shifter to obtain signals having frequencies equal to the sum and difference of the irequencies of said fixed and variable frequency oscillators, first and second filtering means connected'to said first and second combining means, respectively, for passing one of the signals derived from said combining means, both of said filtering means being arranged to pass signals in the same'freqliency band, a phase detector, means forap 'plying the signals from said first and second filtering means to said'phase detector whereby an initial indication of the difference in' phase of sai'd'sig'nals 'may be obtained, and means for disconnecting said second filtering means from saidphase detector and connecting said circuit under test between said second fiitering means and said phase detector, whereby the adjustment of saidphase shifter to obtain an indication corresponding to said initial indication is a measure of the phase shift introduced by said circuit.

CLYDE E. INGALLS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

